Flowability is an important characteristic for all types of powders in industrial processes, including, for example: the manufacture of pharmaceuticals; powder metallurgy; the powder injection molding (PIM) of metal, ceramic and/or composite powders; the formulation of energetic materials; and other processes and applications involving the flow, mixing, dispersion, or fluidization behavior of powders. Flowability is often characterized by the flow rate of a specific mass or volume of powder that passes through a funnel with specified cone angle and aperture [Wanibe, Y.; Itoh, T. New Quantitative Approach to Powder Technology; J. Wiley: New York, 1998, Ch. 10.]. A related method is to vary the size of a funnel aperture to determine the minimum diameter through which the powder will freely flow [a) Taylor, M. K.; Ginsburg, J.; Hickey, A. J.; Gheyas, F. “Composite method to quantify powder flow as a screening method in early tablet or capsule formulation development.” AAPS Pharm Sci Tech, 2000, 1(3) article 18. [http://www.aapspharmscitech.org/pt0103/pt010318/pt010318.pdf last accessed Feb. 2, 2004. b) Gioia, Alberto “Intrinsic flowability: a new technology for powder-flowability classification.” John Morris Scientific. http://203.147.186.54/html/Hanson/flodex_report.htm last accessed Feb. 9, 2004.]. Another dynamic approach is to put a powder sample in a drum that rotates at a constant rate and measure the frequency with which the powder avalanches down the side of the drum [“Aero-Flow Powder Flowability Analyzer Model 3250” TSI Incorporated http://www.tsi.com/powder/products/3250/3250.htm last accessed Feb. 2, 2004.].
A static parameter known to correlate with flowability is the angle of repose of a powder pile, which reflects whether the particles cling together to form a steep pile or slide off one another to form a low mound [Bose, A. Advances in Particulate Materials; Butterworth-Heinemann: Boston, 1995, 300-302.]. Typically, the angle of repose parameter is measured by allowing powder to flow freely through a funnel onto a flat surface to form a pile. The angle between the flat surface and slope of the pile is the angle of repose. Dynamic tests of powder flowability may use specialized equipment, such as a computer controlled Jenike shear cell [Puri, V. M.; Ladipo, D. D. U.S. Pat. No. 6,003,382, Dec. 21, 1999.], and may require samples on the order of 100's of grams. By contrast, the angle of repose measurement requires no specialized equipment and can be adapted to a scale utilizing only a few grams of powder. While both angle of repose and shear flow tests generally differentiate powders based on their flow characteristics, the exact correlation between the tests is not known.
The flowability, or fluidity, of a powder depends on many factors. For example, Table 1 (taken from Rimai, D. “Particle-Substrate Interactions: Microscopic Aspects of Adhesion” NexPress Solutions LLC. Rochester N.Y.) lists both sample dependent and condition dependent factors.
TABLE 1Variables that affect powder flow.1External Factors Powder or Particle VariablesInfluencing Powder BehaviorParticle sizeFlow rateSize distributionCompaction conditionShapeVibrationSurface textureTemperatureCohesivityHumiditySurface coatingElectrostatic chargeParticle interactionAerationWear or attrition characteristicsTransportation experiencePropensity to electrostatic chargeContainer surface effectsHardnessStorage timeStiffnessStrengthFracture toughness1“The Nature of Powders” http://www.freemantech.co.uk/pages/natofpow/nature-powders.html last accessed Feb. 2, 2004.
At a microscopic level, the theory of powder flow is based on theories about adhesion, with the dominant theory being that of Johnson, Kendall, and Roberts (JKR) [Johnson, K. L.; Kendall, K.; Roberts, A. D. Proc. R. Soc. London: Part A, 1971, 324, 301.]. Basic assumptions of the theory include: (1) elastic deformation of spherical particles that vary the contact region, (2) a small contact radius compared to the particle radius, (3) only local interactions in the region of contact are considered—no long range forces are considered to act on the particles [Rimai, D. “Particle-Substrate Interactions: Microscopic Aspects of Adhesion” NexPress Solutions LLC. Rochester N.Y., http://www.clarkson.edu/projects/fluidflow/courses/me537/Rimai—1.pdf last accessed Feb. 2, 2004.]. The JKR theory has been applied to the study of contact surfaces between alkylsilane-coated aluminum and polymers (motivated by the importance of adhesion in the performance of composite materials) [Emerson, J. A.; Giunta, R. K.; Miller, G. V.; Sorensen, C. R.; Pearson, R. A. “The effect of surface contamination on cohesive forces as measured by contact mechanics.” Mat. Res. Symp. 2000, 629, 871-876.].
The cohesive force acting between two polymer surfaces is described as dispersive (non-polar, hydrophobic interactions). In contrast, for metal powders such as aluminum the cohesive forces are predominantly polar. The polar forces arise from hydroxyl group surface termination and physisorbed water molecules on the metal oxide layer that covers most metal surfaces (similar in nature to the well known hydrogen bond formation in molecular chemistry). A realized object of the present invention is to reduce the polar cohesion between metal powder particles by replacing surface hydroxyl groups with hydrophobic alkylsilanes. The prospect of using a hydrophobic surface-modifying agent to improve flow has been broached in a number of patents that describe the use of substituted chlorosilanes and substituted alkoxysilanes [a) Yukiko Ozaki, Satoshi Uenosono, Kuniaki Ogura, Iron-based powder composition for powder metallurgy having higher flowability and higher compactibility and process for production thereof, U.S. Pat. No. 6,503,445, issued Jan. 7, 2003. b) Yukiko Ozaki, Satoshi Uenosono, Kuniaki Ogura, Iron-based powder composition for powder metallurgy excellent in flowability and compactibility and method, U.S. Pat. No. 5,989,304, issued Nov. 23, 1999. c) Takaaki Kohtaki, Masaaki Taya, Masami Fujimoto, Developer containing insulating magnetic toner flowability-improving agent and inorganic fine powder, U.S. Pat. No. 5,547,796, issued Aug. 20, 1996. d) Tetsuya Kobayashi, Haruo Fujii, Motoi Katoh, (Yokohama, JP), Tatsuya Kobayashi, Toshiaki Miyashiro, Naoki Enomoto, Akihiko Uchiyama, Yoshiro Saito, Developing apparatus using one component toner with improved flowability, U.S. Pat. No. 5,307,127, issued Apr. 26, 1994. e) Helmut Knorre, Joachim Fischer, Gerhard Pohl, Process for preventing caking and obtaining flowability of alkali chlorides and salt mixtures thereof U.S. Pat. No. 4,107,274, issued Aug. 15, 1978. f) Helmut Knorre, Joachim Fischer, Gerhard Pohl, Preventing caking and obtaining flowability of alkali chlorides and salt mixtures thereof, U.S. Pat. No. 4,051,228, issued Sep. 27, 1977.]
In the direct reaction of chlorosilanes with hydroxy terminated metal surfaces, the expected products are the tethered or surface bound silanes and HCl. In the literature on modifying aluminum surfaces with alkylsilanes, frequent use is made of the condensation reaction of a silanol with a surface hydroxyl group to eliminate water and form a Si—O—Al linkage. The silanols are usually generated by hydrolysis of either chlorosilanes or alkoxysilanes. Volatile dialkyldichlorosilanes have been applied to metal oxide powders using a gas/solid reaction [Siegmar Laufer, Waldemar Roy, Process for the hydrophobization of higher dispersed oxides, U.S. Pat. No. 3,873,337, issued Mar. 25, 1975.]. Much of the work in the literature is directed to improving the bonding between aluminum and epoxies or polymers [a) Zhang M. C.; Kang E. T.; Neoh K. G.; Tan K. L “Surface modification of aluminum foil and PTFE film by graft polymerization for adhesion enhancement” Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2001, 176(2), 139-150. b) Abel, M-L.; Rattana, A.; Watts, J. F. “Interaction of Epoxy Analogue Molecules with Organosilane-Treated Aluminum: A Study by XPS and ToF-SIMS” Langmuir, 2000 16(16), 6510-6518. c) Lee, I.; Wool, R. P. Thin Solid Films 2000, 379(1-2), 94-100.] and [Underhill, P. R.; DuQuesnay, D. L. “The reaction of water with abraded aluminum surfaces as studied by Fourier transform infrared spectroscopy” App. Surf Sci. 1999, 141, 138-140.]. Some pretreatments are reported to increase the number of surface hydroxyl groups available to react with the surface modifying silane [a) Kabayashi, G. S.; Donnelly, D. J. Boeing Co Report No. DG-41517, 1974. b) Boeing Process Specification BAC 5810, February 1991.]. Other pretreatments create Lewis acid sites on the alumina-like surface of the aluminum metal to increase the binding strength of surface modifying reagent [Puri, V. M.; Ladipo, D. D. U.S. Pat. No. 6,003,382, Dec. 21, 1999.]. In addition to alkoxy- and chlorosilanes, other surface modifiers have been tried, such as acid anhydrides [Schultz, J.; Lavielle, L.; Cane, A.; Comien, P., J. Mater. Sci. 1989, 24, 4363.]. However, the prior art does not disclose or suggest the direct reaction of chlorosilanes with hydroxy terminated metal oxide surfaces to improve the flow characteristics of metal powders such as aluminum, magnesium, titanium, aluminum-magnesium powders, and the like.